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TEXTBOOK OF BOTANY 



environments, such as X-rays, would cause new kinds of variations, 

 whereas they merely increase the frequency of those that occur in nature. 

 This point is further illustrated by the mutations that have occurred 

 in the Boston fern and its varieties growing in greenhouses where the 

 environment is comparatively uniform, though not as uniform as that in 

 a tropical rain forest. The sword fem {Nephrolepis exaltata), a native 

 of Florida and the American tropics, has been cultivated in greenhouses 

 and as a house plant for more than a centurv. In 1895 a florist near Bos- 

 ton discovered among his sword ferns a mutant which became known 



Fig. 218. The cultivated Boston fern (upper left) and five of its many mutant 

 varieties, all of which originated from mutations in vegetative cells. 



as the Boston fern {Nephrolepis exaltata bostoniensis) , and also as the 

 best fem among house plants. Since 1895 more than 200 mutant varieties 

 of the Boston fern are known to have come into existence. Only one of 

 these new varieties has spores that germinate. One variety reproduces 

 by viviparous budding of the leaves. Some of the varieties survive when 

 exposed continuously to the dry air of our homes, which in winter is 

 usually drier than desert air; others can be kept alive only in a saturated 

 atmosphere. The leaves differ greatlv in size and fomi (Fig. 218). Some 

 varieties have leaves several feet long; the smallest varietv has leaves 

 scarcely more than an inch in length. The simplest leaves are once- 

 pinnate and plain. The more complex leaves are 5- to 6-pinnate with vari- 



